1. Field of the Invention
This invention pertains to the field of electrical resistors generally, and specifically to processes used to alter or adjust the value of such resistors.
2. Description of the Related Art
As with any technology, electrical resistors have evolved in many different ways. Materials, processes and applications have all varied and usually improved with time. The processes used to produce electrical resistors have, but for a few very expensive operations, been limited to relatively low cost, rapid operations.
Early methods for production of resistors involved the application of various forms of carbon such as lampblack or graphite to suitable substrates. Exemplary of these early resistors are U.S. Pat. Nos. 1,771,236, 2,041,213 and 2,068,113 assigned to the assignee of the present invention. Since the materials were of low cost and applied via difficult to control methods such as brushing or spraying, the resistance values varied from resistor element to resistor element. As illustrated in the U.S. Pat. No. 1,771,236 patent, resistance values were adjusted using scraping blades which were hand manipulated.
The need for higher reliability and higher operating temperatures in a small, low cost package fueled the development of newer, more robust materials. Exemplary of these materials is the ruthenium cermet materials illustrated in U.S. Pat. No. 3,304,199, also assigned to the assignee of the present invention. This material, a ruthenium based material pioneered by the assignee and adopted worldwide as the industry standard yet today, offers a very high temperature material capable of surviving great extremes of power, temperature and environment.
Part of the ruggedness of the ruthenium based material comes from the combination of ceramics, glasses and metals that make up the composition. It is generally referred to as a Cermet, from contracting the words CERamic and METal. This material can be customized to resistance values ranging from a fraction of an ohm to many megohms, while only requiring a very tiny space upon a substrate.
The new cermet materials revolutionized the electronics industry and opened up applications never before possible. Unfortunately, these materials, like their predecessors, are not precisely reproducible to exact resistance values. Some variation is introduced when the materials are applied (usually by screen printing). Variation may also be introduced during manufacture and during the very high temperature firing processes used to form the finished resistors.
In order to adjust these newer electrical resistors to a final, more exact value, excess material is typically applied. Then, after all variable processes are completed, excess material is trimmed, or removed, from the resistor. In the prior art, this very rugged material is removed with such equipment as specially hardened milling and routering bits, sand blast equipment, and, of more recent fame, laser equipment. Even chemical methods such as acid etching have been considered. Regardless of the equipment used for removal, the end goal is the same. Removal of the right amount of material to leave a resistor of the desired value is the objective.
Further refinement of the trimming processes has led to at least a limited understanding of the events and consequences associated with each removal method. As this understanding has progressed, there have been a number of attempts at defining a better trim pattern to use for removal of the right amount of resistor material. Exemplary of these efforts are U.S. Pat. Nos. 4,403,133 and 5,043,694. In these patents, lasers equipped with modern controllers are used to remove complex patterns of material from resistors. The patterns used are difficult to generate without expensive equipment, and the calculations necessitated by these patterns are difficult. Further, the precision required limits those inventions to computer controlled laser trimming equipment.
One particularly demanding application for electrical resistors is in circuits or environments where large electrical surges may be applied to the electrical resistor. Survival of the resistor during these surges demands a high quality component free from defects that might lead to destructive failure. U.S. Pat. No. 4,528,546 discusses this concern in some detail, but offers a solution of only limited utility.
Prior art FIGS. 1 to 3 are used to illustrate this in detail. Each figure illustrates a simple block type resistor, similar to that illustrated in U.S. Pat. No. 4,528,546. Each of the figures then illustrates a different type of laser trim cut known in the prior art. Electrical terminals are formed and shown as 1 and 2, and may be formed as taught in U.S. Pat. No. 4,528,546, incorporated herein by reference. Typically, this is accomplished by screen printing a conductive composition upon a ceramic substrate, the substrate which is not illustrated in these figures for simplicity sake. A resistor 3 is then formed to interconnect terminals 1 and 2.
FIG. 1 illustrates a simple plunge cut 10 into resistor 3. A controller (not shown) is used to measure the resistance between terminals 1 and 2, and this measured value is then compared with a desired value. The end of the plunge cut, illustrated as point 12, may be computed and then the cut made. However, for precise resistors, the cut is normally made while the resistance is being monitored. When the resistance reaches a desired value, the laser is turned off, so that no further removal of resistor material occurs.
While this method of trimming is fairly simple, a very large voltage gradient will exist across the trim region. The gradient is a result of the redirection of current which occurs as a direct result of the trim. From FIG. 1, looking at lines of current flow 4, it is apparent that the current crowds into the region surrounding point 12. That region will heat very unevenly, and may destructively fail. Additionally, the voltage gradient that exists during a surge condition may cause arcing to occur across the trim line 10 in an area indicated by arrows 5. Such arcing will also lead to destruction of the resistor.
A better type of cut is illustrated in FIG. 2, where the effects of current crowding are reduced somewhat. However, calculations for making the cut are more difficult, and the cut is more time consuming. In addition, the risk of destructive arcing still exists near points 5.
FIG. 3 illustrates a scan cut 36 combined with a plunge cut 30. Here, current flow 4 runs parallel to the scan cut 36, and no localized current crowding exists. This cut is simpler to calculate, since the resistance need only be initially measured and compared with the desired value and the current width. The desired width is then directly calculated and the trim made at the appropriate location. Plunge cut 30 prevents current from flowing through resistor segments 7 and 9. Unfortunately, the full voltage developed between terminals 1 and 2 will be present across cut 30, and will relatively easily arc across from points 5.